1. Field of the Invention
The present invention relates to a multiwavelength simultaneous monitoring circuit employing arrayed-waveguide grating preferably used as a wavelength meter in optical communication networks using wavelength division multiplexing (WDM) technologies, or as a wavelength discriminator in a stabilizing circuit for wavelength division multiplexing optical sources.
2. Description of Related Art
Optical sources used in WDM networks are chiefly laser diodes whose oscillation wavelengths vary with aging or changes in the ambient temperature. Accordingly, it is necessary to measure the wavelengths of a number of laser diodes simultaneously and accurately.
A conventional multiwavelength monitoring circuit that monitors individual wavelengths of a WDM signal carries out wavelength discrimination by scanning the center transmission wavelength of a scanning optical filter (for example, a scanning Fabry-Perot interferometer) on time basis, and thus converting wavelength errors into time domain values, the wavelength errors corresponding to the differences between the wavelength of the WDM signals and the center wavelength of the optical filters.
FIG. 1 shows a configuration of a conventional multiwavelength monitoring circuit (T. Mizuochi, et al., "622 Mbit/s-Sixteen-Channel FDM Coherent Optical Transmission System Using Two-Section MQW DFB-LDs", The transactions of the Institute of Electronics, Information and Communication Engineers of Japan, B-I, Vol. J77-B-1, No. 5, pp. 294-303, 1994).
In this figure, a reference optical signal R and a WDM signal W are multiplexed through an optical coupler 71, and injected into a scanning Fabry-Perot interferometer 72. The scanning Fabry-Perot interferometer 72 carries out scanning using a sawtooth wave (FIG. 2(a)) generated by a sawtooth generator 76 synchronized with an oscillator 75, and an optical signal whose center wavelength coincides with a center transmission wavelength of the Fabry-Perot interferometer 72 is received by a photodetector 73. The output pulses (FIG. 2(b)) of the photodetector 73 are differentiated by a differentiator 78 to detect peak positions of the output pulses (FIG. 2(c)). The sampling pulses (FIG. 2(d)) are generated by a sampling circuit 79 at the peak positions of the output optical pulses in FIG. 2(b). The sampling pulses and the output of the oscillator 75 (FIG. 2(e)) are inputted into a coherent detector 80 whose output is inputted into a sample-and-hold circuit 81. Since the sawtooth wave and the output signal of the oscillator 75 are synchronized, phases of the output signal of the oscillator 75 can be detected by the sampling pulses. The sample-and-hold circuit 81 holds the detected phases, thus producing an error signal as shown in FIG. 2(f). A selector 74 sequentially outputs relative error signals between the center transmission wavelength of the scanning Fabry-Perot interferometer 72 and the wavelengths of the reference optical signal R and the WDM signal W.
The error signal associated with the reference optical signal R is added by an adder 77 to the sawtooth wave outputted from the sawtooth generator 76, and is applied to the scanning Fabry-Perot interferometer 72, so that the positions of the output pulses of the photodetector 73 associated with the reference optical signal R are controlled to be locked at a correct position. Thus, the center transmission wavelength of the scanning Fabry-Perot interferometer 72 can be stabilized using the wavelength of the reference optical signal R, thereby achieving temperature compensation function for variations in the ambient temperature.
On the other hand, the error signals associated with individual wavelengths of the WDM signal W is negatively fed back to respective optical sources of the WDM signal W to control the injection currents or temperature of the optical sources, thereby locking the wavelengths of the WDM signal W.
The conventional scanning Fabry-Perot interferometer can be implemented in a rather simple optical circuit, although a mechanism for scanning the cavity length with piezoelectric device is required. Furthermore, the scanning Fabry-Perot interferometer has an advantage in that wavelength variations can be monitored in a wide range at a desired resolution by appropriately setting its center transmission wavelength and bandwidth.
It is assumed in the conventional scanning Fabry-Perot interferometer that the displacement of the piezoelectric device and the center transmission wavelength of the interferometer are directly proportional to the voltage applied to the piezoelectric device. The actual displacement of the piezoelectric device, however, is not directly proportional to the applied voltage, but exhibits hysteresis characteristics as illustrated in FIG. 3A. Accordingly, to set the center transmission wavelengths which correspond to the displacement of the piezoelectric device at a fixed interval, it is necessary to apply corrected voltages V.sub.2 '-V.sub.5 ' as illustrated by broken lines in FIG. 3B instead of applying equally separated voltages V.sub.1 -V.sub.6.
Thus, the scanning of the center transmission wavelengths cannot be achieved correctly by a linear waveform scanning voltage such as a sawtooth wave in the conventional interferometer. As a result, in the conventional configuration, in which both the sampling and scanning are in synchronism with the same clock pulses, accurate wavelength discrimination in a wide wavelength range is difficult, and hence, it is impossible to achieve accurate monitoring of a WDM signal containing light signals of multiple wavelengths separated at a given interval.